The present invention is directed to a method in a communication device, such as a selective call receiver, and more particularly to a communication device capable of optimizing reception of simulcast and non-simulcast signals.
When designing a communications system, it is often desired to cover an area larger than can be economically covered by a single transmitter site. In such cases, multiple transmitter sites are employed, each transmitting substantially the same data on substantially the same channel, in a process known as simulcasting. Due to differences in propagation delays and other factors, a receiver in the coverage area may receive signals from two or more transmitters at slightly different times, leading to a form of distortion known as Simulcast Delay Spread (SDS) distortion. Under certain conditions this distortion may become severe and corrupt the received data to an unacceptable degree.
Receiver modifications to reduce the effects of SDS distortion are known in the art; however, these modifications tend to degrade static (i.e., non-simulcast) sensitivity, adjacent channel selectivity, or other desirable receiver performance parameters. Conversely, methods of optimizing the receiver to achieve maximum static sensitivity tend to degrade the receiver""s performance in the presence of SDS distortion.
Since it is difficult to simultaneously optimize a receiver for best SDS distortion and static sensitivity performance, a need exists for a method which can reliably discern between a simulcast and static channel at a receiver. Additionally, while it is well known that simulcast reception is predominantly a strong signal phenomenon, simulcast reception can occur in weaker signaling conditions as well. When the signal levels of the individual paths that combine to cause the simulcast distortion are strong (20-30 dB or more above the static sensitivity threshold), an error floor exists when the differential delay between paths (which causes a beat note effect) is as small as xc2xc of a symbol period. However, if the signal levels of the individual paths are not sufficiently above the static sensitivity threshold, then the composite signal can fall below the static sensitivity threshold for relatively long periods of time causing the error rate to go well beyond the strong signal error floor and the error correction capability. Therefore, since in optimizing the demodulator for simulcast conditions usually results in a loss of static sensitivity, doing so at lower signal levels would not be as advantageous as doing so at higher signaling levels. Thus, the need to reliably determine when simulcast optimization will not degrade static sensitivity performance below an acceptable threshold should preferably include an accurate signal strength level test. Likewise, a need to reliably determine when static sensitivity optimization will not degrade simulcast performance below an acceptable threshold should include a probe of the modulation characteristics of the signal being received. If this were available, an adaptable receiver optimized for static sensitivity could be used that employed SDS distortion mitigation methods only when in a simulcast environment, and therefore achieve optimum performance in both static and simulcast environments.